1 W* Invertebrate Paleontology Earth Sciences Division Natural History Museum THE VELIGER © CMS, Inc., 1998 The Veliger 41(2): 157-171 (April 1, 1998) Additions to the Late Paleocene Molluscan Fauna from the Santa Monica Mountains, Los Angeles County, Southern California •'-Tli. j RICHARD L. SQUIRES ^ Department of Geological Sciences, California State University, Northridge, California 91330-8266, USA AND GEORGE L. KENNEDY Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA Abstract. Several previously unreported shallow-marine, warm-water gastropods and bivalves from the upper part of the Santa Susana Formation, east-central Santa Monica Mountains, Los Angeles County, southern California, are described and discussed. The gastropods are Diodora sp. nov.? (Fissurellidae) and Terebralia susana sp. nov. (Potam- ididae). The bivalves are Solena (Eosolen) stantoni (Weaver, 1905) (Solenidae), Martesia sp. (Pholadidae), and Noto- teredoil) sp. (Teredinidae). These mollusks are of late Paleocene (Thanetian Stage) age. For the Pacific Coast of North America, the specimens of Diodora and Martesia represent the earliest records, the specimens of Terebralia the first confirmed record, and the specimens of Nototeredo{1) the first record. The specimens of Solena (Eosolen) stantoni are the best preserved and largest of this species. INTRODUCTION Quarry Canyon, Trailer Canyon, Pulga Canyon, and other The late Paleocene was a time of a large influx of migrant unnamed tributaries (Figure 1). All the localities plot shallow-marine mollusks into the Pacific Coast region of within the upper part of the Santa Susana Formation as North America via circum-global tropical circulation, and mapped by Dibblee (1992). Colburn et al. (1988) and this influx continued on into the early Eocene (Zinsmeis- Colburn (1996) assigned the Paleocene rocks here to the ter, 1983a; Squires, 1988). Upper Paleocene marine rocks Santa Susana Formation, in its broad sense, although oth- are uncommon on the Pacific Coast of North America but er recent workers (e.g., Saul, 1983; Strathearn et al., are well represented in the Palisades Highlands area in 1988) referred to them as the Coal Canyon Formation of the east-central Santa Monica Mountains, southern Cali- Yerkes & Campbell (1979). fornia (Figure 1). Although natural outcrops are scarce The upper part of the Santa Susana Formation in the due to extensive vegetative cover, shallow-marine mol- Santa Ynez Canyon area is a marine unit consisting most- lusks have been found locally, particularly in new expo- ly of olive to gray-green, fine-grained sandstone and silt- sures temporarily uncovered by bulldozer activity during stone, which are bluish gray when unweathered. Mega- the construction of homesites. Most of the specimens of fossils are either in thin lenses or scattered throughout the rare and previously unreported mollusks that are the focus beds. Within the upper part of the formation there are of this paper were collected during the past 15 years by outcrops of coralline-algal limestone, which are white and J. M. Alderson and W. L. Rader, who donated them to resistant. These might represent a single stratigraphic unit local museums. These mollusks are the gastropods Dio- that is present in minor fault blocks and/or landslide dora sp. nov.? and Terebralia susana sp. nov., and the blocks, or they might represent multiple units of similar bivalves Solena (Eosolen) stantoni (Weaver, 1905), Mar- lithology. Previous geologic studies in the area have tesia sp., and NototeredoC?) sp. failed to clarify the stratigraphic relationships. Colburn et The following institutional acronyms are used: CSUN, al. (1988), in a study of the Santa Susana Formation in California State University, Department of Geological the Santa Monica Mountains, considered the algal lime- Sciences, Northridge; LACMIP, Natural History Museum stone to make up a single 10 m-thick marker bed in the of Los Angeles County, Section of Invertebrate Paleon- formation. Strathearn et al. (1988) reported several lenses tology, Los Angeles; and UWBM, University of Wash- of algal limestone. Mack (1993) reported that the algal ington, Thomas Burke Memorial Museum, Seattle. limestones (10 to 30 m thick) apparently represent several stratigraphic levels. Strathearn et al. (1988) and Mack STRATIGRAPHY (1993), however, grouped all of the algal limestones into The mollusks discussed in this report were collected from a single stratigraphic unit in their generalized stratigraph- the area east of Santa Ynez Canyon, in the tributaries of ic columns. Page 158 The Veliger, Vol. 41, No. 2 top not seen Siltstone (120 m) TJ m 3D Coralline- CO > algal • 16897 limestone with CO interbedded <= CO siltstone > (30 m) O 70 Siltstone > 11984,16888, with H 26814 sandstone O interbeds 16869 (thickness unknown) Figure 2 Generalized columnar section showing relative stratigraphic po- sitions of the cited megafossil localities in the upper Santa Su- sana Formation in the Palisades Highlands and vicinity, east- central part of the Santa Monica Mountains, southern California. Recent reconnaissance work by the senior author to resolve the algal-limestone problem in the immediate study area was uninformative because all of the outcrops had been obliterated by extensive housing-tract develop- ment. Pending future information, it seems best to refer to the algal limestone(s) as the "algal-limestone inter- val." Although the exact stratigraphic position of this in- terval is not known because structural complications make it impossible to identify the upper contact of the Santa Susana Formation, the interval is approximately 120 m below the stratigraphically highest outcrops of the Santa Susana Formation in the area (Mack, 1993). The mollusks described in this report were collected Figure 1 from rocks stratigraphically near or within the algal-lime- stone interval, and their positions are shown in Figure 2. Index map showing LACMIP collecting localities in the upper The pholadid bivalve Martesia sp. and the teredinid bi- Santa Susana Formation, Palisades Highlands and vicinity, east- central Santa Monica Mountains, Los Angeles County, southern valve Nototeredo('?) sp. are from a richly fossiliferous California. Base map from Dibblee (1992). lens within the gray-green very fine-grained sandstone at LACMIP loc. 16869 (Figures 1,2). This lens is approxi- mately 20 m below the algal-limestone interval. The fau- na at this locality is much more diverse than those from other units and yielded numerous specimens of the bi- R. L. Squires & G. L. Kennedy, 1998 Page 159 valves Cucullaea mathewsonii Gabb, 1864, Crassatella ed by a few large specimens of Turritella infragranulata unioides (Stanton, 1896), Saulella undulifera (Gabb, Gabb, 1864, (very near to T. i. pachecoensis Stanton, 1869), and Pholadomya (P.) nasuta Gabb, 1864, and the 1896, fide L. R. Saul, personal communication) and some gastropods Turritella infragranulata Gabb, 1864, and small specimens of Tornatellaea pinguis (Gabb, 1864). various species of naticids. A few small specimens of the solenid bivalve Solena (Eosolen) stantoni were also pres- DEPOSITIONAL ENVIRONMENT ent. The two gastropods treated herein are from fine- Colburn et al. (1988) and Colburn (1996) concluded that grained sandstone immediately subjacent to the algal the upper part of the Santa Susana Formation in the Santa limestone and are from slightly higher in the stratigraphic Ynez Canyon area was deposited in a low-energy pro- section than where Martesia and Nototeredo were found. tected bay, no deeper than 40 m, with the bay situated Most of the specimens of the fissurellid gastropod Dio- behind a barrier bar. They based their conclusion on the dora sp. nov.? were found at LACMIP loc. 16888. One following: condition of the megafossils, fine grain size of other specimen of Diodora sp. nov.? was found at LAC- the deposits, presence of well-developed horizontal lam- MIP loc. 11984, and a single specimen of the potamidid inae within the deposits, and lack of sedimentary struc- gastropod Terebralia susana sp. nov. was found at LAC- tures associated with strong wave or current action. Their MIP loc. 26814 in the Quarry Canyon area and below an megafossil evidence consists of articulated bivalve shells, algal limestone that Dibblee (1992) mapped as the only both adult and juvenile specimens of the same species, algal limestone in that area. This specimen of Terebralia and absence of both preferred orientation and current-size susana is somewhat worn. sorting of the shells. They also concluded that branching The largest and best preserved specimens of the solen- calcareous algae developed shoals on the bay floor. id bivalve Solena (Eosolen) stantoni are from gray-green Strathearn et al. (1988) concluded that the upper part muddy silstone in the upper Pulga Canyon area at LAC- of the Santa Susana Formation in the Santa Ynez Canyon MIP loc. 16897. This siltstone appears to be a major in- area accumulated in an unrestricted, muddy, middle-shelf terbed within the algal-limestone interval and, if so, environment near or below storm-wave base and no deep- would be slightly higher stratigraphically than the other er than 70 m. They based their conclusions on the taxo- discussed mollusks. Unfortunately, there have been no nomic composition of the dinoflagellates and benthic fo- detailed stratigraphic sections in the critical area of upper raminifera in the deposits, as well as on the predomi- Pulga Canyon where Dibblee's (1992) geologic